It is of great current interest to develop an aircraft with the ability to cruise at hypersonic velocities up to Mach 25. Aircraft which are capable of such flights pass through different flight regimes during a typical flight, including low, medium, and high Mach number velocities. Such aircraft are subjected to an extremely broad temperature range, from the extremely low temperatures associated with subsonic atmospheric flight to the extremely hot temperatures (e.g., 5000 degrees Fahrenheit) associated with hypersonic atmospheric flight. As a consequence, the aircraft must be designed to withstand such extreme temperatures and velocities.
The development of materials which are capable of operating in such environments has been underway for a considerable time. Carbon-carbon composites have been found to be particularly suitable for such applications, as they can undergo exposure to the temperature and pressure extremes expected with hypersonic aircraft without losing strength or otherwise degrading. However, such composites oxidize when exposed to oxygen, as will be found in the atmosphere. Oxidation effectively burns up structures made from carbon-carbon composites.
To combat the effects of oxidation on carbon-carbon composites, coatings, including silicon carbide (SiC) coatings, have been developed. Coatings of this type have been disclosed in U.S. patents issued to Patten, et al. (U.S. Pat. No. 4,500,602), Holzl (U.S. Pat. No. 4,515,860), Shuford (U.S. Pat. Nos. 4,471,023 and 4,465,777), Honjo et a1. (U.S. Pat. No. 4,405,685), and Chandler et al. (U.S. Pat. No. 4,621,017). Carbon-carbon composite materials which have been coated with such compositions are very stiff, and attempts to increase their flexibility by reducing their thickness also reduces their strength.
Particular concern has been directed toward the various seals which must be used on hypersonic aircraft. Such seals include those for aerodynamic control surfaces, windows, landing gear doors and engine ramp/splitter walls (which duct air into the aircraft's engines). These seals must be designed to withstand the extreme temperatures and velocities while maintaining the requisite flexibility. They should also be capable of preventing the passage of high temperature gases and can, consequently, be more effective if they take the form of a single piece seal. The seal problem is particularly difficult for the engine ramp/splitter wall seals, since leakage of the hot atmospheric gases can severely affect the performance of the aircraft's engines and leakage of pressurizing gases from under the engine ramps is inefficient.
Many of the seal applications in a hypersonic aircraft must also allow for the two sealed surfaces to move with respect to one another, frequently against the direction of the high velocity atmosphere. Carbon fiber seals have long been proposed for use in regenerators for gas turbine engines, as disclosed in United States patents issued to Zeek et al. (U.S. Pat. No. 3,743,008), Siegla (U.S. Pat. No. 3,856,077), Rao (U.S. Pat. No. 3,913,926), Sakaki (U.S. Pat. No. 4,071,076), and French et al. (U.S. Pat. No. 4,183,539). However, the prior art does not show that such seals are intended to move with or against the direction of the flowing gases which they seal.
Attachment of the seal is another aspect of the problem. When the seal is operating in environments whose temperatures can reach 5000 degrees Fahrenheit, it can be very difficult to reliably attach the seal to one of the surfaces it is intended to seal. This is particularly the case when the seal is exposed to a high velocity atmosphere which can quickly oxidize most fasteners.
It is therefore desirable to have seals made from the stiff materials which can both withstand the high temperatures and pressures expected in the operation of a hypersonic aircraft and prevent the leakage of hot gases.